Evaporator devices for volatile substances are very well-known, for diffusing air fresheners, pesticides or similar chemical substances.
Are well known in the market containers containing volatile substances such as insecticide or fragrances that are closed by a semi-permeable membrane. Most of these products include a polyolefin monolithic membrane. Due to high selectivity of polyolefins, and the complexity of some volatile substances (very complex fragrances compositions), these products generally have a quite low evaporation rate and accept a limited number of fragrances with quite simple fragrances compositions (with low quality).
Several intends have been done to try to solve these drawbacks. On one side, the use of microporous films allows a much lower selectivity of the fragrance through the membrane and thus a higher evaporation rate of a higher quality fragrance. Nevertheless, the transport of fragrances through this kind of membrane is by capillarity that is a process that is driven by pressure and the product may leaks in some conditions.
On the other side, transport through monolithic membranes is driven by gradient concentration, so if the fragrance on the external side of the membrane do not evaporate, no more fragrance is transported through the membrane and the fragrances cannot leaks.
Several materials have been proposed but most of them have elastomeric characteristics. This gives the advantage of a higher permeability of fragrances through these materials. The problem with these membranes is that permeation through the membrane is done partially by solubilisation of the liquid in the membrane material, that mean, how much liquid is able to contain a certain volume of membrane material. So, the higher the solubility the higher the permeability. Nevertheless, there is a price to pay for that and that is that the elastomeric membrane material use to swell, that provokes wrinkles on the surface and decrease its mechanical strength.
This negative effect is completely linked to the chemical nature of the volatile substance so the same product with different fragrances may present several different level of this defect.
Additionally, this kind of membrane product use to come with a barrier top aluminium layer that has to be pealed off before first use. In the case of elastomeric materials, these materials use to have already quite lower mechanical resistance vs polyolefins, that could be even lower due to the chemical solubilisation. This may provoke a membrane rupture when aluminium is peeled off.
Finally, in the case of monolithic membranes, permeability use to be related to the amount of surface of the membrane that is in contact with the liquid. This has 2 consequences:
As refill is not completely full, the maximum evaporation surface is never used.
As liquid level decreases with time, the evaporation rate decrease proportionally along device life.